Technetium-99m-labeled receptor-specific small-molecule radiopharmaceuticals: recent developments and encouraging results

Synthesis and evaluation of technetium-99m labelled 1-(2-methoxyphenyl)piperazine derivative for single photon emission computed tomography imaging for targeting 5-HT1A

Quantitative changes in expression level of 5HT_1A are somewhere related to common neurological disorders such as anxiety, major depression and schizophrenia. We have designed EDTA conjugated SPECT imaging probe for localization of 5HT_1A receptor in brain. For designing SPECT probe we have employed the concept of bivalent approach and a homodimeric system with desirable pharmacokinetics of 5HT_1A imaging. ^99mTc-EDHT was also evaluated for its stability through serum stability assay and glutathione challenge experiment. Biodistribution study showed the highest accumulation of radioactivity in kidney which depicted the renal mode of excretion from the body. However in brain the uptake of 1.21% ID per gram was observed in initial 5 min of drug administration. On blocking the receptor this percent get decreased to 0.97% ID per gram. The regional distribution in brain was also performed which showed the accumulation of drug in cerebellum, cortex and hippocampus part, which are already known for 5HT_1A expression. Dynamic study in rabbit is also in support of results derived from biodistribution and blood kinetics experiment. These finding suggest that ^99mTc-EDHT holds promising place for further optimization before nuclear medicine applications in different animal species.

Review on 99mTc radiopharmaceuticals with emphasis on new advancements

Rapid imaging acquisition, high spatial resolution and sensitivity, powered by advancements in solid-state detector technology, are significantly changing the perspective of single photon emission tomography (SPECT). In particular, this evolutionary step is fueling a rediscovery of technetium-99m, a still unique radionuclide within the nuclear medicine scenario because of its ideal nuclear properties and easy preparation of its radiopharmaceuticals that does not require a costly infrastructure and complex procedures. Scope of this review is to show that the arsenal of technetium-99m radiopharmaceuticals is already equipped with imaging agents that may complement and integrate the role played by analogous tracers developed for positron emission tomography (PET). These include, in particular, somatostatin (SST) and prostate-specific membrane antigen (PSMA) receptor targeting agents, and a number of peptide-derived radiopharmaceuticals. Additionally, these recent technological developments, combined with new myocardial perfusion tracers having more favorable biodistribution and pharmacokinetic properties as compared to current commercial agents, may also reinvigorate the prevailing position still hold by technetium-99m radiopharmaceuticals in nuclear cardiology.

188Re(V) Nitrido Radiopharmaceuticals for Radionuclide Therapy

The favorable nuclear properties of rhenium-188 for therapeutic application are described, together with new methods for the preparation of high yield and stable ¹⁸⁸Re radiopharmaceuticals characterized by the presence of the nitride rhenium core in their final chemical structure. ¹⁸⁸Re is readily available from an ¹⁸⁸W/¹⁸⁸Re generator system and a parallelism between the general synthetic procedures applied for the preparation of nitride technetium-99m and rhenium-188 theranostics radiopharmaceuticals is reported. Although some differences between the chemical characteristics of the two metallic nitrido fragments are highlighted, it is apparent that the same general procedures developed for the labelling of biologically active molecules with technetium-99m can be applied to rhenium-188 with minor modification. The availability of these chemical strategies, that allow the obtainment, in very high yield and in physiological condition, of ¹⁸⁸Re radiopharmaceuticals, gives a new attractive prospective to employ this radionuclide for therapeutic applications.

Pharmacokinetic aspects of retinal drug delivery

Drug delivery to the posterior eye segment is an important challenge in ophthalmology, because many diseases affect the retina and choroid leading to impaired vision or blindness. Currently, intravitreal injections are the method of choice to administer drugs to the retina, but this approach is applicable only in selected cases (e.g. anti-VEGF antibodies and soluble receptors). There are two basic approaches that can be adopted to improve retinal drug delivery: prolonged and/or retina targeted delivery of intravitreal drugs and use of other routes of drug administration, such as periocular, suprachoroidal, sub-retinal, systemic, or topical. Properties of the administration route, drug and delivery system determine the efficacy and safety of these approaches. Pharmacokinetic and pharmacodynamic factors determine the required dosing rates and doses that are needed for drug action. In addition, tolerability factors limit the use of many materials in ocular drug delivery. This review article provides a critical discussion of retinal drug delivery, particularly from the pharmacokinetic point of view. This article does not include an extensive review of drug delivery technologies, because they have already been reviewed several times recently. Instead, we aim to provide a systematic and quantitative view on the pharmacokinetic factors in drug delivery to the posterior eye segment. This review is based on the literature and unpublished data from the authors' laboratory.

A contribution to the rational design of Ru(CO)3Cl2L complexes for in vivo delivery of CO

A few ruthenium based metal carbonyl complexes, e.g. CORM-2 and CORM-3, have therapeutic activity attributed to their ability to deliver CO to biological targets. In this work, a series of related complexes with the formula [Ru(CO)3Cl2L] (L = DMSO (3), L-H3CSO(CH2)2CH(NH2)CO2H) (6a); D,L-H3CSO(CH2)2CH(NH2)CO2H (6b); 3-NC5H4(CH2)2SO3Na (7); 4-NC5H4(CH2)2SO3Na (8); PTA (9); DAPTA (10); H3CS(CH2)2CH(OH)CO2H (11); CNCMe2CO2Me (12); CNCMeEtCO2Me (13); CN(c-C3H4)CO2Et) (14)) were designed, synthesized and studied. The effects of L on their stability, CO release profile, cytotoxicity and anti-inflammatory properties are described. The stability in aqueous solution depends on the nature of L as shown using HPLC and LC-MS studies. The isocyanide derivatives are the least stable complexes, and the S-bound methionine oxide derivative is the more stable one. The complexes do not release CO gas to the headspace, but release CO2 instead. X-ray diffraction of crystals of the model protein Hen Egg White Lysozyme soaked with 6b (4UWN) and 8 (4UWN) shows the addition of Ru(II)(CO)(H2O)4 at the His15 binding site. Soakings with 7(4UWN) produced the metallacarboxylate [Ru(COOH)(CO)(H2O)3](+) bound to the His15 site. The aqueous chemistry of these complexes is governed by the water-gas shift reaction initiated with the nucleophilic attack of HO(-) on coordinated CO. DFT calculations show this addition to be essentially barrierless. The complexes have low cytotoxicity and low hemolytic indices. Following i.v. administration of CORM-3, the in vivo bio-distribution of CO differs from that obtained with CO inhalation or with heme oxygenase stimulation. A mechanism for CO transport and delivery from these complexes is proposed.

Synthesis of the first radiolabeled 188Re N-heterocyclic carbene complex and initial studies on its potential use in radiopharmaceutical applications

A novel approach towards the synthesis of radiolabeled organometallic rhenium complexes is presented. We successfully synthesized and analyzed the first (188)Re-labeled N-heterocyclic biscarbene complex, trans-dioxobis(1,1'-methylene-bis(3,3'-diisopropylimidazolium-2-ylidene))(188)rhenium(V) hexafluorophosphate ((188)Re-4) via transmetalation using an air-stable and moisture-stable silver(I) biscarbene complex. In order to assess the viability of this complex as a potential lead structure for in vivo applications, the stability of the (188)Re-NHC complex was tested in physiologically relevant media. Ultimately, our studies illustrate that the complex we synthesized dissociates rapidly and is therefore unsuitable for use in radiopharmaceuticals. However, it is clear that the transmetalation approach we have developed is a rapid, robust, and mild method for the synthesis of new (188)Re-labeled carbene complexes.

Complexes possessing rare "tertiary" sulfonamide nitrogen-to-metal bonds of normal length: fac-[Re(CO)3(N(SO2R)dien)]PF6 complexes with hydrophilic sulfonamide ligands

Tertiary sulfonamide nitrogen-to-metal bonds of normal length are very rare. We recently discovered such a bond in one class of fac-[Re(CO)3(N(SO2R)(CH2Z)2)](n) complexes (Z = 2-pyridyl) with N(SO2R)dpa ligands derived from di-(2-picolyl)amine (N(H)dpa). fac-[M(CO)3(N(SO2R)(CH2Z)2)](n) agents (M = (186/188)Re, (99m)Tc) could find use as radiopharmaceutical bioconjugates when R is a targeting moiety. However, the planar, electron-withdrawing 2-pyridyl groups of N(SO2R)dpa destabilize the ligand to base and create relatively rigid chelate rings, raising the possibility that the rare M-N(sulfonamide) bond is an artifact of a restricted geometry. Also, the hydrophobic 2-pyridyl groups could cause undesirable accumulation in the liver, limiting future use in radiopharmaceuticals. Our goal is to identify a robust, hydrophilic, and flexible N(CH2Z)2 chelate framework. New C2-symmetric ligands, N(SO2R)(CH2Z)2 with (Z = CH2NH2; R = Me, dmb, or tol), were prepared by treating N(H)dien(Boc)2, a protected diethylenetriamine (N(H)dien) derivative, with methanesulfonyl chloride (MeSO2Cl), 3,5-dimethylbenzenesulfonyl chloride (dmbSO2Cl), and 4-methylbenzenesulfonyl chloride (tolSO2Cl). Treatment of fac-[Re(CO)3(H2O)3](+) with these ligands, designated as N(SO2R)dien, afforded new fac-[Re(CO)3(N(SO2R)dien)]PF6 complexes. Comparing the fac-[Re(CO)3(N(SO2Me)dien)]PF6 and fac-[Re(CO)3(N(SO2Me)dpa)]PF6 complexes, we find that the Re(I)-N(sulfonamide) bonds are normal in length and statistically identical and that the methyl (13)C NMR signal has an unusually upfield shift compared to that in the free ligand. We attribute this unusual upfield shift to the fact that the sulfonamide N undergoes an sp(2)-to-sp(3) rehybridization upon coordination to Re(I) in both complexes. Thus, the sulfonamide N of N(SO2R)dien ligands is a good donor, even though the chelate rings are conformationally flexible. Addition of the strongly basic and potentially monodentate ligand, 4-dimethylaminopyridine, did not affect the fac-[Re(CO)3(N(SO2tol)dien)]PF6 complex, even after several weeks. This complex is also stable to heat in aqueous solution. These results indicate that N(SO2R)dien ligands form fac-[Re(CO)3(N(SO2R)dien)]PF6 complexes sufficiently robust to be utilized for radiopharmaceutical development.

Synthesis of multidentate ligands with amido or amino donor groups for the preparation of rhenium and technetium radiopharmaceuticals

A new method to prepare novel semi-rigid multidentate ligands containing nitrogen atom, to coordinate with rhenium and technetium, was established. The method was based on formylation of substituted anilines, followed by Mannich reaction with glycine and paraformaldehyde. The method was very promising to design ligands of various molecular structures (L₁–L₅) to coordinate with rhenium metal ions. The complexes were prepared through ligand exchange with the complex ReOCl₃(PPh₃)₂, giving new complex of the structure ReOCl₃L_(1–5). The prepared ligands and complexes were identified by the use of UV–vis, and infrared absorption spectrometric techniques, elemental analysis, molecular weight determination by depression of freezing point. These ligands were labeled with ^99mTc pertechnetate, and the labeling efficiency of the complexes was measured using a well type scintillation gamma counter equipment and obtained a good yield.

APPLICATION OF TECHNETIUM AND RHENIUM IN NUCLEAR MEDICINE

Technetium and Rhenium are the two lower elements in the manganese triad. Whereas rhenium is known as an important part of high resistance alloys, technetium is mostly known as a cumbersome product of nuclear fission. It is less known that its metastable isotope 99mTc is of utmost importance in nuclear medicine diagnosis. The technical application of elemental rhenium is currently complemented by investigations of its isotope 188Re , which could play a central role in the future for internal, targeted radiotherapy. This article will briefly describe the basic principles behind diagnostic methods with radionuclides for molecular imaging, review the 99mTc -based radiopharmaceuticals currently in clinical routine and focus on the chemical challenges and current developments towards improved, radiolabeled compounds for diagnosis and therapy in nuclear medicine.

Fluorine- and rhenium-containing geldanamycin derivatives as leads for the development of molecular probes for imaging Hsp90

Heat shock protein 90 (Hsp90) is an ATP-dependent molecular chaperone responsible for protein quality control in cells. Hsp90 has been shown to be overexpressed in many human cancers. This has prompted extensive research on Hsp90 inhibitors as novel anticancer agents and, more recently, the development of molecular probes for imaging Hsp90 expression in vivo. This work describes the development of various fluorine-containing and rhenium-containing geldanamycin derivatives as leads for the development of corresponding (18)F-labeled and (99m)Tc-labeled PET and SPECT probes for molecular imaging of Hsp90 expression. All compounds were evaluated in an in vitro ATPase activity assay using Hsp90 isoform Hsp82p. Fluorobenzoylated geldanamycin derivative 5 displayed comparable inhibitory potency like parent compound geldanamycin.

A molecular imaging primer: modalities, imaging agents, and applications

Molecular imaging is revolutionizing the way we study the inner workings of the human body, diagnose diseases, approach drug design, and assess therapies. The field as a whole is making possible the visualization of complex biochemical processes involved in normal physiology and disease states, in real time, in living cells, tissues, and intact subjects. In this review, we focus specifically on molecular imaging of intact living subjects. We provide a basic primer for those who are new to molecular imaging, and a resource for those involved in the field. We begin by describing classical molecular imaging techniques together with their key strengths and limitations, after which we introduce some of the latest emerging imaging modalities. We provide an overview of the main classes of molecular imaging agents (i.e., small molecules, peptides, aptamers, engineered proteins, and nanoparticles) and cite examples of how molecular imaging is being applied in oncology, neuroscience, cardiology, gene therapy, cell tracking, and theranostics (therapy combined with diagnostics). A step-by-step guide to answering biological and/or clinical questions using the tools of molecular imaging is also provided. We conclude by discussing the grand challenges of the field, its future directions, and enormous potential for further impacting how we approach research and medicine.

Kinetics of formation for lanthanide (III) complexes of DTPA-(Me-Trp)2 used as imaging agent

Diethlenetriamine-N,N,N'N''N''-pentaacetic acid (DTPA)-bis (amide) analogs have been synthesized and evaluated as a potential biomedical imaging agents. Imaging and biodistribution studies were performed in mice that showed a significant accumulation of DTPA analogs in brain. The stability and protonation constants of the complexes formed between the ligand [DTPA-(Me-Trp)(2)] and Gd(3+), Eu(3+), and Cu(2+) have been determined by pH potentiometry (Gd(3+), Eu(3+)) and spectrophotometry (Cu(2+)) at 25 °C and at constant ionic strength maintained by 0.10 M KCl. The kinetic inertness of Gd [DTPA-(Me-Trp)(2)] was characterized by the rates of exchange reactions with Zn(2+) and Eu(3+). In the Eu(3+) exchange, a second-order [H(+)] dependence was found for the pseudo-first-order rate constant [k(0) = (4.5 ± 1.2) × 10(-6)/s; k(1) = 0.58 ± 0.1 /M/s, k(2) = (6.6 ± 0.2) × 10(4) /M(2)/s, k(3) = (4.8 ± 0.8) × 10(-4) /M/s]. In the Eu(3+) exchange, at pH <5.0, the rate decreases with increasing concentration of the exchanging ion. At physiological pH, the kinetic inertness of [DTPA-(Me-Trp)(2)] is more inert than GdDTPA(2-), the most commonly used MRI contrast agent (t(1/2) = 127 h). High kinetic stability is an important requirement for the Gd complexes used as contrast enhancement agents in magnetic resonance imaging.

Synthesis and biological characterization of novel 2-quinolinecarboxamide ligands of the peripheral benzodiazepine receptors bearing technetium-99m or rhenium

Potential receptor imaging agents based on Tc-99m for the in vivo visualization of the peripheral benzodiazepine receptor (PBR) have been designed on the basis of the information provided by the previously published structure-affinity relationship studies, which suggested the existence of tolerance to voluminous substituents in the receptor area interacting with 3-position of the quinoline nucleus of 2-quinolinecarboxamides 5. In the first step of the investigation, the stereoelectronic features of the above-indicated receptor area were also probed by means of 4-phenyl-3-[(1-piperazinyl)methyl]-2-quinolinecarboxamide derivatives bearing different substituents on the terminal piperazine nitrogen atom (compounds 6a-f). The structure-affinity relationship data confirmed the existence of a tolerance to bulky lipophilic substituents and stimulated the design of bifunctional ligands based on the 4-phenyl-3-[(1-piperazinyl)methyl]-2-quinolinecarboxamide moiety (compounds 6h,j,k,m). The submicromolar PBR affinity of rhenium complexes 6j,m suggests that the presence of their metal-ligand moieties with encaged rhenium is fairly compatible with the interaction with the PBR binding site. Thus, in order to obtain information on the in vivo behavior of these bifunctional ligands, (99m)Tc-labeled compounds 6h,k were synthesized and evaluated in preliminary biodistribution and single photon emission tomography (SPET) studies. The results suggest that both tracers do not present a clear preferential distribution in tissues rich in PBR, probably because of their molecular dimensions, which may hamper both the intracellular diffusion toward PBR and the interaction with the binding site.

Exploring biologically relevant chemical space with metal complexes

Altering biological processes with small synthetic molecules is a general approach for the design of drugs and molecular probes. Medicinal chemistry and chemical biology are focused predominately on the design of organic molecules, whereas inorganic compounds find applications mainly for their reactivity (e.g. cisplatin as a DNA-reactive therapeutic) or imaging properties (e.g. gadolinium complexes as MRI diagnostics). In such inorganic pharmaceuticals or probes, coordination chemistry in the biological environment or at the target site lies at the heart of their modes of action. However, past and very recent results suggest that it is also worth exploring a different aspect of metal complexes: their ability to form structures with unique and defined shapes for the design of 'organic-like' small-molecule probes and drugs. In such metal-organic compounds, the metal has the main purpose to organize the organic ligands in three-dimensional space. It is likely that such an approach will complement the molecular diversity of organic chemistry in the quest for the discovery of compounds with superior biological activities.

Molecular modeling of hexakis(areneisonitrile)technetium(I), tricarbonyl eta5 cyclopentadienyl technetium and technetium(V)-oxo complexes: MM3 parameter development and prediction of biological properties

Genetic algorithms (GA) were used to develop specific technetium metal-ligand force field parameters for the MM3 force field. These parameters were developed using automated procedures within the program FFGenerAtor from a combination of crystallographic structures and ab initio calculations. These new parameters produced results in good agreement with experiment when tested against a blind validation set. To illustrate the utility of these new force field parameters, quantitative structure-activity relationship (QSAR) models were developed to predict the P-glycoprotein uptake (log10 VI) of a series of hexakis(areneisonitrile)technetium(I) complexes and to predict their biodistribution. The log10 VI QSAR model, built using a training set of 16 Tc(I) isonitrile complexes, exhibited a correlation between the experimental log10 VI and 5 simple descriptors as follows: r2 = 0.94, q2 = 0.93. When applied to an external test set of six Tc(I) isonitrile complexes, the QSAR preformed with great accuracy q2 = 0.78 based on a leave-one-out cross-validation analysis. Further QSAR models were developed to predict the biodistribution of the same set of Tc(I) isonitrile complexes; a QSAR model to predict hepatic uptake exhibited a correlation between the experimental log10(Blood/Liver) with six simple descriptors as follows: r2 = 0.97, q2 = 0.96. A QSAR model to predict renal uptake exhibited a correlation between the experimental log10(Blood/Kidney) and six simple descriptors as follows: r2 = 0.85, q2 = 0.82. When applied to the external test set the QSAR models preformed with great accuracy, q2 = 0.78 and 0.56, respectively.

A novel 99mTc-labeled testosterone derivative as a potential agent for targeting androgen receptors

With an insight that ligands possessing a N2S2 tetradentate array of donor atoms serve as ideal bifunctional chelating agents (BFCA) in the radiolabeling of target-specific agents, 5-hydroxy-3,7-diazanonan-1,9-dithiol (DAHPES) with a derivatizable substituent in the form of a hydroxyl group in the backbone was synthesized. The preparation of a steroid conjugate via coupling of this BFCA with testosterone-3-(O-carboxymethyl) oxime and the subsequent radiolabeling of the conjugate under optimized conditions with 99mTc, the ideal diagnostic radionuclide in nuclear medicine procedures, are reported. The immunoreactivity of the radiolabeled conjugate was demonstrated in a study using anti-testosterone antibodies, wherein the radiolabeled conjugate exhibited significant binding with antiserum to testosterone. Cell-uptake studies in DU145 prostate carcinoma cell line bearing androgen receptors (ARs) and comparison with AR non-bearing breast carcinoma cell line revealed the specific binding of the steroidal moiety with the testosterone receptor.

99mTc-labeled annexin V fragments: a potential SPECT radiopharmaceutical for imaging cell death

INTRODUCTION

Annexin V is a protein that binds to phosphatidylserine exposed on dying cells. The phosphatidylserine-specific sequence is attributed to a chain on the N-terminal of annexin consisting of 13 amino acid sequence. Radiolabeled annexin V is used for imaging apoptosis.

METHODS

With an aim to synthesize a probe that can detect cell death akin to annexin V but smaller in size, annexin-13 fragments were derivatized to contain cysteine, cysteine-cysteine and histidine in their sequence at N terminal and were labeled with (99m)Tc via nitrido and carbonyl precursors. The (99m)Tc-labeled annexin-13 derivatives were characterized by HPLC and studied for their stability. In vitro and in vivo studies were carried out in apoptotic HL-60 cells and fibrosarcoma tumor-bearing Swiss mice, respectively.

RESULTS

The (99m)Tc complexes were formed in high yields and were found to be stable. HPLC pattern of (99m)Tc nitrido complex of cysteine-cysteine-annexine 13 (CC-Anx13) and (99m)Tc carbonyl complex of histdine-annexin 13 (H-Anx13) revealed the formation of single species. In vitro cell uptake studies with (99m)Tc nitrido complex of cysteine-cysteine-annexin 13 fragment showed 6.5% uptake in apoptotic HL-60 cells. The uptake was found to be specific on testing with apoptotic HL-60 cells. Biodistribution studies of (99m)Tc nitrido complex with CC-Anx13 in fibrosarcoma tumor-bearing Swiss mice revealed optimum tumor uptake of 0.52 (0.17) %ID/g at 1 h pi.

CONCLUSION

(99m)Tc(N)-CC-anx13 showed specific uptake in apoptotic tumor cells and warrants further evaluation.

Thiol- and thioether-based bifunctional chelates for the {M(CO)3}+ core (M = Tc, Re)

By analogy to the recently described single amino acid chelate (SAAC) technology for complexation of the {M(CO)3}+ core (M = Tc, Re), a series of tridentate ligands containing thiolate and thioether groups, as well as amino and pyridyl nitrogen donors, have been prepared: (NC5H4CH2)2NCH2CH2SEt (L1); (NC5H4CH2)2NCH2CH2SH (L2); NC5H4CH2N(CH2CH2SH)2 (L3); (NC5H4CH2)N(CH2CH2SH)(CH2CO2R) [R = H (L4); R = -C2H5 (L5). The {Re(CO)3}+ core complexes of L1-L5 were prepared by the reaction of [Re(CO)3(H2O)3]Br or [NEt4]2[Re(CO)3Br3] with the appropriate ligand in methanol and characterized by infrared spectroscopy, 1H and 13C NMR spectroscopy, mass spectrometry, and in the case of [Re(CO)3(L2)] (Re-2) and [Re(CO)3(L1)Re(CO)3Br2] (Re-1a) by X-ray crystallography. The structure of Re-2 consists of discrete neutral monomers with a fac-Re(CO)3 coordination unit and the remaining coordination sites occupied by the amine, pyridyl, and thiolate donors of L2, leaving a pendant pyridyl arm. In contrast, the structure of Re-1a consists of discrete binuclear units, constructed from a {Re(CO)3(L1)}+ subunit linked to a {Re(CO)3Br2}- group through the sulfur donor of the pendant thioether arm. The series of complexes establishes that thiolate donors are effective ligands for the {M(CO)3}+ core and that a qualitative ordering of the coordination preferences of the core may be proposed: pyridyl nitrogen approximately thiolate > carboxylate > thioether sulfur > thiophene sulfur. The ligands L1 and L2 react cleanly with [99mTc(CO)3(H2O)3]+ in H2O/DMSO to give [99mTc(CO)3(L1)]+ (99m)Tc-1) and [99mTc(CO)3(L2)] (99mTc-2), respectively, in ca. 90% yield after HPLC purification. The Tc analogues 99mTc-1 and 99mTc-2 were subjected to ligand challenges by incubating each in the presence of 1000-fold excesses of both cysteine and histidine. The radiochromatograms showed greater than 95% recovery of the complexes.